Filter United VIVA21 6-Stage Complete Replacement Filter Set (V21RF6S12PH)

One $99.99 box rebuilds the entire VIVA21 filter train, including the alkaline remineralization cartridge that is $49.99 bought alone.

This is the full cartridge set for the VIVA21 6-stage system: the 5-micron sediment filter, both 5-micron extruded carbon blocks, the 50 GPD TFC reverse osmosis membrane, the coconut shell post-carbon filter, and the alkaline remineralization cartridge, at $99.99 against a $139.99 compare-at. The 5-stage equivalent is $79.99. Bought piecemeal, the alkaline cartridge alone is $49.99 and Filter United rates it for 18 months of normal household use. The brand's own change schedule is sediment and carbon about every 12 months and the membrane and inline carbon every 2 to 3 years, which is why it also quotes roughly $40 to $80 a year in consumables rather than the full set price annually.

The cross-compatibility claim is the part to read carefully. Filter United states these cartridges fit all standard under-sink reverse osmosis systems and names APEC, Express Water, and iSpring by name on its own pages. That is a self-declared fit claim, not a certified compatibility list, and there is no published cross-reference chart, so if you are buying these for a system that is not a VIVA21, measure your housings and confirm with Filter United before ordering. The same certification caveat applies here as on the system: the WQA Gold Seal and NSF 58 and 372 language is the brand's claim, published without a certificate number, and we could not verify it independently.

The clean fit is a VIVA21 owner doing an annual or biennial rebuild, where one $99.99 box replaces the whole train and there is no fit question at all. The second fit is a cross-shopper with another under-sink RO system who is willing to check housing sizes themselves before ordering.